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Forum Index : Windmills : blade weight comparing

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catchnthe-))))

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Joined: 03/01/2009
Location: United States
Posts: 24

Posted: 01:23am 21 Jan 2009

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Hello all,
1st post. reading a post yesterday about stalling of turbine.
Years ago I helped build racing motorcycles. For flat track
types of racing the Magneto was removed from crankshaft to allow High REV'S of motor and a 12 volt battery was used to supply voltage to coil.

Any way I'm curious..
there are so many different materials
used for prop blades. I realize that balancing is very critical for vibration control and a smooth running turbine.
Has anyone done testing of weight of propeller in relation to stalling. (wood versus say aluminum or pvc) Once a large amount of prop weight is started it would seem to allow it to run smoother during gusting and slower wind.

Thanks
Chuck

Freedom allows us a lot "Please don't abuse."

SparWeb

Senior Member

Joined: 17/04/2008
Location: Canada
Posts: 196

Posted: 05:04pm 21 Jan 2009

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Folks tend to use the term "stalling" pretty liberally when talking about windmills, so you may be misled on what they mean.
If there is vibration, turbulence, rapid swivelling of the rotor around the tower, these are loads that stress the blades, and generally increase wear-and-tear. Some energy is robbed from the system, so any bad match between propeller and alternator is worsened.

But when the subject of stalling comes up, usually people are talking about how much energy the blades can efficiently capture from the wind, in relation to how much power the alternator requires to be turned. So if the alternator is too small, the blades over-power it, run fast, and the blade airfoils have a small angle of attack to the wind.

If the alternator is rather large, and the blades work hard to turn it, then the angle of attack of the airfoil can get high. If that angle of attack gets too high, then airfoil drag increases, robbing power from the system. If the generator is too large, the blades simply won't produce enough power to turn it any faster than some modest little speed, and folks go wondering why the power reaches a "plateau". This isn't actually as bad as it sounds - if stalling happens only in strong winds you have a natural speed control that prevents your windmill from exploding!

Lots of other things happen, so there's plenty of stuff to observe and discuss, making it a popular topic, here.

Welcome to the Backshed!

Steven T. Fahey

oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686

Posted: 08:30pm 21 Jan 2009

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Hehehe,
Yes there is stall, stall and stall. Depending on our backgrounds will depend on what we think it means.

To a farmer, it is where he keeps the horse, to the church it is where you keep the choir, to the vendor, it is where you sell your stuff, to the driver it is when you dump the clutch and stop, to the mediators it is when the talks get no-where etc etc.

I use the term loosely to describe a condition somewhere between car stalling (putting power in, but not getting movement out), and Sparwebs definition which describes a combination of aspect ratios, angles of attack and incident wind (I think).

If a mill is showing signs of losing TSR as the wind picks up because of load induced drag (alt to big), the average joe will probably see that as varying forms of stall. An aeronautical type will see it as a complex mathematical case, and conclude that the angle of attack versus the aspect ratio of the blade and the incident wind velocity...... that it is indeed not in stall in the true sense... and mostly he'd be right.

As there are more average joe's around, it seems unavoidable that the weight of perception will be contrary to the purist view.... I guess I'm just another evil joe ( unrepentant sinner ) in this respect.

Sparweb is quite right, and justified in feeling that the rest of us are just not getting it.

Blade weight:
I have found that blade weight is mostly a non-event. Heavy blades will respond slightly slower to changes in wind speed, but not enough to get excited about. Light blades do just fine. The only thing that matters in this respect seems to be balancing the set. My individual blades weights are different, but static balancing seems to leave me with a 4meter set without the slightest sign of vibration... even at silly speeds when I have inadvertently let the load off.

...........oztules

Village idiot...or... just another hack out of his depth

GWatPE

Senior Member

Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 11:10pm 21 Jan 2009

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When you carve blades from natural materials like wood, it is unlikely the mass distribition will be the same.

The method I use for balancing wooden blades that are physically alike, but different weight is:

Weigh each blade and record, balance each blade on a knife edge, and mark the centre of mass. Compare this mark on all blades. I now use layers of 2pak sealer to balance the blades. I take the heaviest blade as a reference, to match to. I add layers of sealer [painted on by brush] to the light blades to increase the mass, while noting any change in the mass distribution. As the weights become closer, I add sealer only to the part of the blade that brings the centre of mass closer to the same point. In the end, all blades will weigh the same and will have the same centre of mass. This has worked well on my wooden blades that have had up to 100g weight difference before balancing. Once all the blades are the same, it is just a matter of ensuring the correct tip to tip distances and hub bolting positions. I prefer to not have to worry which blade goes where with reassembly. If all the blades are the same, then this makes it easier. This balancing process may take a while. I may need to repeat this same technique with the painting, to balance down to the 2g tolerance that I have, in a system where the blades only weigh 700g each.

The lower the blade mass the faster the blades will achieve the optimum speed. In an extreme case, Heavy blades may slow windmill response to a level where a wind gust has already passed and the windmill does not respond to it. There is no inertia in the wind, and if the blade is accelerating, but with a delay, then as soon as the windspeed reduces the blade speed reduces. It will not capture some of the peak wind that has just been past.

If you get the opportunity to compare 2 windmills of similar rotor size, one with light blades and one with heavy blades, in close proximity, the light blade windmill produces more power.

A well balanced mill will also start turning in less wind.

Gordon.

become more energy aware

catchnthe-))))

Newbie

Joined: 03/01/2009
Location: United States
Posts: 24

Posted: 08:07pm 29 Jan 2009

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Thanks:
oztules
Being Raised on a farm seen the stall.
Teaching my kids to drive a stick shift been there. Dido..

So thanks for the input on weight proportions to blades.
I live in a "can be a very windy area" 70 mph last few days. common to have 15 to 20 mph. 29 mph as this is being typed.

My concern not only with Furling mentioned on the forum here is Safety to my project.

This should probably be a separate post. here goes
Can total # of blades be used as a governing,as a second control method.

Just another question from the newbie.
Thanks again for your comparisons I had to chuckle though. on first two.

Thanks
Chuck

Freedom allows us a lot "Please don't abuse."

oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686

Posted: 04:46am 30 Jan 2009

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[quote]This should probably be a separate post. here goes
Can total # of blades be used as a governing,as a second control method. [/quote]

In theory yes, in practice it's not really workable. Coming off a farm, you may likely have had a pumper windmill with lots of metal blades. (mines got 18 I think) If you let these go flat out in a good wind, you will find they seem to give up at about 200 rpm, but operation speed is about 5-120rpm. (and it's only about 5' diameter) The wake of the blade in front meses up the clear wind for the one following it, and it gets to the point where none of them have good air, and the power drops off.

Sounds like a good system.... but, trying to use low rpm for a high torque pump is one thing, trying to make a generator efficient at low rpm is another. It would be a very difficult task to carve out all the blades, and then build an alternator to match.

Gravity furling is very reliable if set up carefully. We are in the "roaring forties" here, and high wind speeds are normal. The furling easily protects the 4m mill I have, but getting the furling right is critical. Once done it is pretty idiot proof.

3 blades is enough to carve I reckon,(even using the chainsaw method) and the higher RPM because of the low blade count, gives you a chance to build a decent alternator without too much trouble.

If Sparweb drops by, he may decide to expand further on the aerodynamics side in particular.... he knows his stuff.

..........oztules

Village idiot...or... just another hack out of his depth

whiskey

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Joined: 19/06/2007
Location: Ireland
Posts: 21

Posted: 12:23pm 30 Jan 2009

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TSR is a much abused term and an excuse for poor blade design. How often is it said that a wind turbine stalls at high speeds because the blades are not powerful enough to turn the large dual rotor PMG.

Once the blade swept area is sufficient enough to provide input power, the problem is one of incorrect blade profile and every home brew blade out there is carved with good startup and mid range. When it stalls at higher wind speeds all the excuses and band aids such as using more resistance in line or widening the air gap are used.

The blade stalls because it does not have enough lift at higher wind speeds, or to be correct, higher Reynolds numbers. Designing a blade not to stall with a given load at a certain Reynolds number is done all the time successfully and for many years. They are called airplane propellers!....we just extract power from the wind with our propeller.

The reason why most home brew wind turbine blades wont reach the full potential of the PMG is because most profiles used are for aircraft use and not suitable for the low Reynolds numbers we must design to, and they also have too high a solidity.

If you want proof of blades that do work in low Reynolds numbers, look at human flight and gliders. Both in the regime we want. But carving one takes lots patience and careful math. A wing or propeller is the same thing, one just flies at a different speed to the other and it is wing profile sections we look at for our propellers because that is the easiest way for our brains to get around the problem.

TSR in itself is not a good way to look at blade design or stall. As the wind speed increases and assuming you carved a blade with distributed twist i.e angle of attack (and geometric pitch) there will be a point along the blade that gives the best spot for lift, with higher wind speed the part of the blade providing lift will go outboard of the blade towards the tip. At higher speeds where all the lift is generated at or near the tip, this is where all your hard work on designing the blade will pay off, and of vital importance is to implement the Larabee method of optimising the blade planform and reducing tip losses. Tip loss is a big problem for every wing and every blade. Poor tip loss can sometimes be heard be ear at high wind speeds, and that is not a good sign, it means you are loosing electrical power.

Carving any blade with a roundy front and pointy back is no good for efficient power harvest. You must decide on a profile and stick to it. It is possible to mix more than one profile in a blade if you know what your doing. This has benefits once you can see the effect of the various profiles. I have a simulation package to aid me in design and also use a home brew wind tunnel for testing profile sections and top loss.

So, Stall means many things and depends on where you apply it. Some excellent people to look for on the web are Larabee, Atkins, Somers and the human flight work. All factual and real world application of complex math and testing. You'll learn a lot from those clever guys.

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